Why are digital lenses measured in 35mm numbers?

Digital lenses are measured in 35mm equivalent focal lengths, though they do not correspond exactly to a 35mm film camera's focal length. The reasoning behind this is similar to the reasoning behind why gasoline engines are still measured in horsepower: It's a measure with which everyone is familiar. Digital SLRs, in particular, are marketed heavily to professional photographers. Because the 35mm SLR was for the better part of a century the standard for professional photographers, digital camera manufacturers wanted to make certain their devices were understandable by that market.

The focal length itself is the distance between the center of the lens and its focal point. This number indicates the nature of the lens in regard to whether it's a telephoto model, a wide-angle model or somewhere in between. Longer focal lengths mean a lens is capable of seeing for a long distance. Shorter focal lengths are generally wide-angle lenses suitable for shooting in confined spaces or for special effects. Most digital SLRs come equipped with a zoom lens which is capable of offering different focal lengths for different purposes.

Your old SLR lenses may well fit on your digital SLR. Each manufacturer has their own mounting. In many cases, one may simply take the lens off their 35mm film camera and attach it directly to their digital camera. If this is the case, however, the focal length aspects come even more into play.

The actual focal length of the lens, if one that was formerly used on a 35mm is transferred to a digital SLR, will vary as the actual surface to which the light is exposed differs in size from 35mm film. 35mm film was larger across the diagonal than is the image sensor used by digital SLRs. Your instruction manual will tell you the difference, but in general, 35mm film is between 1.5 and 1.6 times larger across the diagonal than are the CCD chips used in digital SLRs. This means you can calculate the actual focal length of a lens used on a 35mm camera by multiplying it by whatever the specific difference happens to be. For instance, a 50mm lens that was used on a 35mm camera will function as a 75mm lens on a digital SLR with a sensor 1.5 times larger and as an 80mm lens on a digital SLR with a sensor 1.6 times larger.

Keep in mind that most digital SLRs have features such as autofocus that may not translate between lenses transferred from film cameras. The method of light-metering may also not be as reliable when using such lenses, as well. Overall, however, the fact that focal lengths can be translated from one to another means that all those lenses you purchased for your 35mm film camera do not have to go to waste!

• Some lenses to consider are:

the telephoto zoom lens Canon 75-300 f/4-5.6 III USM,

the all in one zoom lens Pentax SMC DA 18-250mm f3.5-6.3 ED AL(IF)

or the fixed focal length Nikon AF NIKKOR 180mm f/2.8D IF-ED